In this patent application a new type of stent is described with a unit cell, having a negative spring rate and a bistable function. Such a unit cell can also be used in other medical applications. This means that it has two configurations in which it is stable without the need for an external force to hold it in that shape. The unit cell is formed using at least two different sections. One section is less pliable than the other one and acts a relatively rigid support that hinders the shape change of the more pliable section in one direction. In the other direction the pliable section can be deformed, but because of the opposing force from the rigid section, the stability of the pliable or flexible section is strongly increased.
External forces in a direction perpendicular to the most pliable section are distributed to the rigid section and the cross section of the pliable section is merely loaded in compression mode. This makes the construction much stronger than prior stents. In prior stents, all struts have generally the same cross section and mechanical properties and are merely used in the bending mode.
The construction of a stent, based upon this unit cell results in an apparatus, that can easily be elastically compressed around the balloon by finger pressure.
Below a certain critical diameter, the present stent snaps further to a stable, smallest diameter, thus holding the deflated balloon firmly on to the surface of the catheter, with an insertion diameter that is as small as possible. An additional sheath is not required, but may be used for extra safety.
After the stent has been brought into the patient's body at the proper axial position, the balloon can be inflated until the stent reaches its critical elastic equilibrium diameter. Slightly above this diameter the stent automatically expands further to its final largest diameter, where it reaches its maximum stability against radial pressure. The design enables a constant length large expansion ratio, a reliable expandability and/or a small surface ratio.
A further embodiment of this invention is the possibility of a kind of stepwise expanding stent with a range of stable diameters.
Another part of the invention is a stent with several external diameters along its length, to adapt as good as possible to the shape of the body cavity where it is placed.
Another part of the invention is the possibility to modify the stress and strain pattern in the unit cell by means of a heat treatment in such a way, that the force displacement characteristic of this unit cell becomes asymmetrical or even exhibits a monostable instead of a bistable function, either with the expanded diameter or the collapsed diameter being the most stable condition.
Another embodiment of the invention is the modification of the geometry of the cross section of some struts to achieve the symmetric or asymmetric bistable or monostable force against displacement characteristics of a unit cell.
Another part of the invention is the use of one or more unit cells in other medical applications such as, for example, an expander or a clip, either to spread a body cavity open or to clamp or hold a body part or some body tissue.
The invention is also directed to the use of the inventive stents in conjunction with inventive expander rings to join together two vessels.
The invention is also directed to a bistable valve having a snap-action bipositional unit cell and uses for the same, in particular, to prevent incontinence.
The invention is also directed to multistable cells and their use in medical devices.